A Study On Dynamics And Chemistry Of Groundwater Around Stagnation Points

The non-uniform spatial distribution of flow velocity is a common phenomenonin aquifers. In the past, the non-uniform flow field is always attributed toheterogeneity of the aquifer. In fact, even if the aquifer is homogeneous and isotropic,this non-uniform distribution of flow velocity could exist. For instance, due to theundulation of water table, the groundwater flow systems of different orders coulddevelop in homogeneous and isotropic drainage basin, and there are stagnant zoneswith extremely low velocity among the flow systems.(Tóth,1963; Fetter,2001; Jianget al.,2011).In the past several decades, most researchers discussed the development ofstagnation points in groundwater flow systems based on the assumption ofsteady-state flow. Unfortunately, there is little research on the development ofstagnation points under transient flow conditions. Tóth(1963,1980) assumed thatsubstances such as oil and gas could accumulate in stagnant zones where groundwaterflow systems meet. Nonetheless, due to the lack of quantitative research on substancesaccumulation around stagnation points, the issue of whether substances aroundstagnation points accumulate around stagnation points is still controversial. In order toanswer these questions, the theoretical and applied research is presented as follows.First of all, the evolution of stagnation points under transient flow conditions isconducted. We modify Tóth’s (1980) complex basin model by changing the upperboundary from a specified-head boundary to a specified-flux boundary. The variationof the groundwater flow systems before and after tunnel excavation is analyzed vianumerical simulation. According to the research, the hieratically nested flow systemscan also develop with a specified-flux boundary and local stagnation points existamong them. After tunnel excavation, the groundwater flow field changessignificantly. The local flow systems gradually disappear, and finally there are onlyregional flow systems left. Corresponding to the evolution of groundwater flowsystems, the stagnation points change and disappear, whose evolution pattern can beused to briefly delineate the evolution of the groundwater flow field. Based on theoretical analysis, a profile model of the Jianfengling Tunnel located in Chongqingis constructed to study the evolution of stagnation points in the groundwater systems,whose results confirm the theoretical results.We also conduct a study on chemistry of groundwater around stagnation points.Three models, conservative tracer transport model, accumulative tracer transportmodel and decaying tracer transport model, are constructed, respectively. Bycomparing the three models via numerical simulation, the mechanism of traceraccumulation around stagnation points is studied. The results show that the substancescould not accumulate around the stagnant point for the conservative tracer transportmodel. However, the tracer concentration around the stagnation point is higher for theaccumulative tracer transport model while the tracer concentration around thestagnation point is lower for the decaying tracer transport model.The results of this paper deepen the understanding of the evolution law ofgroundwater systems under transient flow caused by artificial mining and themechanisms of tracer accumulation under steady-state flow conditions. Moreover, theresults could provide guidance for water resources development, petroleum andmineral exploitation.